Giardia is a widespread zoonotic intestinal parasite and is one of the ten major parasites of humans, causing significant acute and chronic diarrheal disease. Due to the lack of concerted research efforts, giardiasis has been designated a World Health Organization (WHO) neglected disease. To proliferate and colonize the small intestine, trophozoites first find suitable sites for attachment using flagellar motility, and then avoid peristaltic flow by attaching to the villi with the ventral disc, a unique microtubule structure. Te ventral disc is critical for pathogenesis in that it mediates reversible attachment to the intestinl microvilli via an undefined mechanism. Our primary focus is to evaluate the functions of ventral disc substructures that are required for overall disc conformational changes leading to parasite attachment in vivo and in vitro. We will first determine the contributions of flagellar motility an lateral crest (LC) contact to early stages of attachment (Aim 1). We will then correlate the various disc conformational states occurring in late stage attachment with movements of substructures (Aim 2), and determine the contributions of these substructures to generating disc conformations required for attachment (Aim 2). Finally, we will define the stages disc biogenesis with respect to the assembly of disc substructures after mitosis, when two dorsal daughter discs are assembled and the parental ventral disc is disassembled, and during excystation, when two new daughter discs are rapidly formed. This work will inform our analyses of disc structure and function (Aims 1-2). We will also test the role of posttranslational tubulin modifications in sequentially recruiting disc-associated proteins to the assembling disc (Aim 3). Drugs affecting flagellar motility, ventral disc structure, disc biogenesis, or disc conformational dynamics may directly or indirectly decrease trophozoite attachment in the host, and thus limit the initiation o extent of infection.